1. Field of the Invention
The present invention relates to a ceramic electronic part, and more particularly to a ceramic electronic part having a structure in which an external terminal, for being mounted to, for example, a base, is disposed on a surface of a ceramic electronic part element so as to be in electrical conduction with an internal electrode disposed in ceramic.
2. Description of the Related Art
Some ceramic electronic parts include external terminals for being mounted to, for example, bases. An example of such ceramic electronic parts including external terminals are shown in FIGS. 8A, 8B, and 8C, wherein metallic external terminals 55 are bonded with a bonding material 56, such as solder, an electrically conductive adhesive, or an electrically conductive paste, onto both end surfaces of a chip-type ceramic electronic part element 54 where external electrodes 53 are formed so as to be in electrical conduction with internal electrodes 52 disposed in ceramic 51.
In another example shown in FIG. 9, metallic external terminals 55 are bonded through a bonding material 56, such as solder, an electrically conductive adhesive, or an electrically conductive paste, onto a ceramic electronic part element 64 formed by stacking a plurality of chip-type electronic part elements 54 upon each other, where external electrodes 53 are formed so as to be in electrical conduction with internal electrodes 52 disposed in ceramic 51.
In general, the external terminals 55 shown in FIGS. 8A and 8C, having widths WE approximately equal to width W of the ceramic electronic part element 54 (64), are often used.
The use of such external terminals does not become a serious problem when the ceramic electronic parts whose external terminals 55 are bonded to the ceramic electronic part element 54 (64) are small. However, when the ceramic electronic parts are large with, for example, a length L or a width W (see FIGS. 8A to 8C and FIG. 9) equal to or greater than 10 mm, they are often used with a relatively high voltage and a large current, so that in order to make resistance values low the widths WE (see FIG. 8A) and thicknesses T (see FIGS. 8B and 9) of the external terminals 55 must be made large.
In such a case, a noticeable amount of stress is produced due to a difference between the linear expansion coefficients of the ceramic electronic part element 54 (64) and those of the external terminals 55, so that a considerably large stress is exerted onto both end portions of the external terminals 55. For this reason, during, for example, a heat cycle test of the ceramic electronic parts, cracks C are produced in the ceramic electronic part element 54 (64), particularly at the sides where the temperature is low, as shown in FIG. 10.
To overcome the above-described problem, the present invention is able to provide a highly reliable ceramic electronic part with an external terminal, which eliminates the problem of cracks being produced in a ceramic electronic part element when the ceramic electronic part is large.
To this end, according to a first form of the present invention, there is provided a ceramic electronic part comprising:
a ceramic electronic part element constructed so that an internal electrode is disposed in ceramic, with a portion of the internal electrode being exposed from an end surface of the ceramic electronic part element; and
an external terminal being a metallic plate joined to the end surface of the ceramic electronic part element so as to be in electrical conduction with the internal electrode exposed from the end surface of the ceramic electronic part element;
wherein a width of a portion of the external terminal that is joined to the end surface of the ceramic electronic part element is equal to or less than a width of the internal electrode disposed in the ceramic electronic part element.
When the width of the portion of the external terminal that is joined to the ceramic electronic part element is equal to or less than the width of the internal electrode, and the ceramic electronic part element has a complex structure including the ceramic and the internal electrode (metal), an external electrode can be joined to a portion having a linear expansion coefficient closer to the linear expansion coefficient of the metal than the linear expansion coefficient of a portion including only the ceramic. Therefore, it is possible to reduce cracking as a result of reducing the amount of stress produced in a heat cycle test.
In the present invention, the phrase xe2x80x9cjoining an external terminal to an end surface of a ceramic electronic part elementxe2x80x9d is to be broadly interpreted as also meaning, for example, directly joining an external terminal to an end surface of a ceramic electronic part element, or joining an external terminal onto an end surface of a ceramic part element through, for example, a bonding material, such as solder, an electrically conductive adhesive, or an electrically conductive paste, and/or joining the external terminal to an external electrode formed on the end surface of the ceramic electronic part element in conductive connection with the internal electrode.
According to a second form of the present invention, the joining portion of the external terminal may be joined to the end surface of the ceramic electronic part element so as to be positioned within an area of the end surface of the ceramic electronic part element where the internal electrode is exposed when viewed in a plane.
When an external electrode is joined to an end surface of the ceramic electronic part element so that the joining portion of the external terminal is positioned within the area of the end surface of the ceramic electronic part element where the internal electrode is exposed when viewed in a plane, the external terminal can be reliably joined to a portion having a linear expansion coefficient closer to the linear expansion coefficient of a metal than the linear expansion coefficient of a portion including only ceramic. Therefore, the present invention can provide more satisfactory practical results.
According to a third form of the present invention, when a width of a portion of the external terminal that is joined to the end surface of the ceramic electronic part element is equal to or less than a width of the internal electrode disposed in the ceramic electronic part element, or when the joining portion of the external terminal is joined to the end surface of the ceramic electronic part element so as to be positioned within an area of the end surface of the ceramic electronic part element where the internal electrode is exposed when viewed in a plane, the ceramic electronic part element may be formed by stacking a plurality of ceramic electronic part elements upon each other.
Here, it is possible to use a type of ceramic electronic part known as a stacked-type ceramic electronic part formed by stacking a plurality of ceramic electronic part elements upon each other. Even in this case, it is possible to reduce the amount of stress generated in a heat cycle test in order to reduce cracking.
According to a fourth form of the present invention, when a width of a portion of the external terminal that is joined to the end surface of the ceramic electronic part element is equal to or less than a width of the internal electrode disposed in the ceramic electronic part element, or when the joining portion of the external terminal is joined to the end surface of the ceramic electronic part element so as to be positioned within an area of the end surface of the ceramic electronic part element where the internal electrode is exposed when viewed in a plane, or when the ceramic electronic part element is formed by stacking a plurality of ceramic electronic part elements upon each other, a width of at least a portion of the external terminal other than the joining portion that is joined to the ceramic electronic part element may be made larger than the width of the internal electrode disposed in the ceramic electronic part element.
When the width of at least a portion of the external terminal other than the joining portion that is joined to the ceramic electronic part element (that is, the width of a portion of the external terminal that is joined to a land of a base when the ceramic electronic part is to be mounted to the base) is made larger than the width of the internal electrode disposed in the ceramic electronic part element, the area of the external terminal joined to the land can be made larger to achieve more reliable mounting, making it possible to design the ceramic electronic part with more freedom while maintaining its reliability.
According to a fifth form of the present invention, when a width of a portion of the external terminal that is joined to the end surface of the ceramic electronic part element is equal to or less than a width of the internal electrode disposed in the ceramic electronic part element, or when the joining portion of the external terminal is joined to the end surface of the ceramic electronic part element so as to be positioned within an area of the end surface of the ceramic electronic part element where the internal electrode is exposed when viewed in a plane, or when the ceramic electronic part element is formed by stacking a plurality of ceramic electronic part elements upon each other, or when a width of at least a portion of the external terminal other than the joining portion that is joined to the ceramic electronic part element is made larger than the width of the internal electrode disposed in the ceramic electronic part element, the form of the ceramic electronic part element when viewed in a plane may be such that a length (width) W of a side A of the end surface to which the external terminal is mounted is greater than a length L of a side B perpendicular to the side A.
When the form of the ceramic electronic part element when viewed in a plane is such that the length (width) W of the side A of the end surface to which the external terminal is mounted is larger than the length L of the side B that is perpendicular to the side A, stress is often produced between the external terminal and the ceramic electronic part element, so that cracks are often produced. However, when the present invention is applied to such a ceramic electronic part, cracking can be reduced, making the ceramic electronic part highly reliable. Therefore, it is particularly worthwhile to apply the present invention to such a ceramic electronic part.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.